The present disclosure relates generally to power converters using an active-clamp flyback topology and control methods thereof, and, more particularly, to control methods that operates a power converter using an active-clamp flyback topology in one of several operation modes.
Flyback power converters have been widely adopted in the power supplies of home appliances, computers, battery chargers for example. To further improve the efficiency of a flyback power converter, an active-clamp topology has been introduced, using an active-clamp circuit to replace a snubber, which is commonly used to consume the energy stored by the leakage inductance of a primary winding of a transformer in the flyback power converter. A power converter using an active-clamp flyback topology is named an ACF power converter in short. ACF power converter is well known to have outstanding power efficiency when a load of the ACF power converter is heavy. However, the power efficiency deteriorates seriously when the load is light, substantially due to the significant circulated current continuously going back and forth through a primary winding of the ACF power converter.
Texas Instruments introduces UCC28780, a controller used in an ACF power converter. UCC28780 is capable of operating in one of four operation modes, basically in response to the state of the load of the ACF power converter. The circuit application datasheet of USCC28780, however, still shows a bleeder resistor connected in parallel with a capacitor of an active-clamp circuit, to slowly release the energy accumulated on the capacitor. Obviously, as the bleeder resistor acts as an energy eater, USCC28780 does completely employ the benefit of the active-clamp circuit.
Furthermore, system designers of conventional ACF power converters usually confront the difficulties in dealing with electromagnetic interference (EMI) and audible noise.